The present invention relates to miniature, flexible, controlled-impedance transmission line cables comprising an elongate pair of transversely separated, side-by-side conductors for transmitting high-frequency signals in computer and other comparable applications.
Electrical conductor pairs suitable for the transmission of high-frequency signals must have a number of critical characteristics which are not important for conductors used for lower frequency transmissions. These characteristics include reliable uniformity of transverse spacing between the conductors, and uniformity of dielectric constant in the regions transversely separating the conductors, so that capacitance between the conductors is reliably predictable.
Moreover, the lengths of the two conductors, and their resultant delays, must be identical so that the signals carried by the respective conductors arrive at their destinations in synchronization. Since such conductor pairs are often twisted helically to resist adverse effects of external magnetic fields, achieving equal electrical length of the conductors requires that the respective helical twists have a uniform length, referred to as "lay length"; otherwise, when cutting a twisted pair of conductors to a desired length, one conductor may be longer than the other even though they are cut to length in unison.
Moreover, the foregoing uniform parameters must remain stable despite subsequent bending or other handling of the conductors during manufacture, operation, and servicing of the equipment. While one might assume that this can readily be accomplished simply by fastening the conductors together in a common outer jacket, this step has presented numerous problems in practice. One problem is the significant increase in cross-sectional area of the conductor pair required to encase it in such a jacket. The cross-sectional area of the conductor pair is increased markedly if a common external jacket is applied to the pair of conductors by extrusion or other means. Such increase in cross-sectional are constitutes a serious disadvantage in attempting to use conductor pairs in high-density applications where literally thousands of such conductor pairs must extend side-by-side within limited confines and be terminated at correspondingly high-density connectors. Moreover, the capacitance and thus characteristic impedance of the conductor pair can be rendered nonuniform by the application of a common outer jacket to the two conductors, particularly by the inadvertent creation of air voids in the region surrounding the two conductors. Even an outer jacket extrusion process, when applied to a pair of side-by-side conductors, cannot reliably fill in all voids surrounding the conductors. Such air voids become a particularly severe problem in equipment where the conductor pairs are immersed in a liquid, such as the coolant fluorinert. Ultimately, such fluid finds its way into such air voids, creating a stability problem because a substantial time period may be required for the liquid to completely fill the voids. Moreover, the cable is periodically separated from the fluid for purposes of servicing or replacing components, causing the liquid to drain, evaporate or diffuse from the voids. Thereafter, when the cable is once more immersed in the liquid, a substantial time period may be required for the liquid to refill the voids and become stable. In the meantime, an unstable period of changing dielectric constants an resultant changing impedances may render the system inoperable.
Alternatively, attempting to dispense with the common outer jacket by bonding respective dielectric layers, immediately surrounding the respective conductors, directly to each other is unsatisfactory because the preferred dielectrics, such as FEP or PTFE, are very difficult to bond reliably with adhesives or solvents. Conversely, if heat bonding is utilized, the dielectric layers would be altered by such bonding at least dimensionally, and in some cases also with respect to their dielectric constants, thereby making it difficult to controllably predetermine the electrical characteristics of the resulting conductor pair.
Many examples of multiple, interconnected electrical conductors and their methods of manufacture exist in the prior art, such as those shown in the following U.S. Pat. Nos.:
3,649,434 PA1 4,131,690 PA1 4,218,581 PA1 4,234,759 PA1 4,368,214 PA1 4,468,089 PA1 4,515,993 PA1 4,541,980
However, none of these suggests a solution to any of the foregoing problems of miniature controlled-impedance transmission lines having transversely separated side-by-side conductors.